To loosen, convey or sieve loose material or for compacting of concrete in molds, apparatus performing a swinging motion are often used, as for instance swing conveyors, swinging sieves, respectively compacting vibrators. The necessary swinging motion is often produced by rotating unbalanced exciters. The unbalanced masses of such exciters are usually arranged in pairs, that is at each end of the shaft of the drive motor in order to avoid tumbling motions. For strictly linear vibrating motions two equal unbalanced masses must be driven in opposite direction synchronously in the same plane which requires special provisions for the synchronous movement. Such swing drives require a relatively large space and are constructed very heavily. In addition, the unbalanced forces are transmitted in their totality over the bearings onto the device which requires correspondingly large dimensioned and expensive bearings.
Besides such unbalanced exciters there exist also electromagnetic linear vibrators as swing drives. In these known electromagnetic linear vibrators the armature swings in the longitudinal direction of a pulsating magnetic field back and forth, whereby, shortly before impinging onto one pole shoe of the stator, it is intercepted by a spring and thrown back until it is intercepted by the oppositely acting second spring to be again accelerated in the direction toward the stator. If the armature moves in the direction of the line of magnetic flux, the air gap is shortened while the pulling force is at the start of the movement very small and at the end of the stroke considerably larger and, therefore, the armature can only carry out very small strokes in the order of about 1 millimeter. For this reason the mass of the armature has to be chosen considerably large, which results in large and heavy vibrators. In addition, the springs must be constructed very strong and stiff. This can be obtained only with metal springs. This in turn will result in a harmonic swinging motion. While a harmonic swinging motion is suitable for some applications, as for instance reciprocating feeders, in other applications, as for instance loosening or compacting material, motions with larger accelerations are better suitable. For this purpose the known linear vibrators are less suitable.